The advantages of providing power assisted steering units in vehicles are well known. Most automobiles and trucks on the market today include power steering units as a convenience to the operator. As vehicle size increases, the necessity for power steering assist increases as well. Indeed, some of the larger trucks on the road today must be equipped with power steering units in order to make comfortable operation feasible with a normal size steering wheel.
Conventional power steering units provide essentially the same degree of assist under all operating conditions. This represents a compromise, because at low speeds and high steering inputs, a large degree of power assist is desirable, but at higher speeds and smaller steering inputs, this same high degree of assist is no longer required. Indeed, at highway speeds, most vehicles require very little power assist. The high level of assist provided by the conventional units at highway speeds results in an undesirable loss of road feel.
U.S. Pat. No. 3,693,747 to Nishikawa assigned to Honda discloses a control apparatus to reduce power assist in response to increasing vehicle speed. This system requires an input corresponding to vehicle speed which, in the embodiment disclosed, is a variable oil pressure signal from a motor driven governor.
Similarily, U.S. Pat. No. 4,434,866 to Duffy assigned to Ford Motor Company discloses a variable assist power steering system. This system provides lesser power assist at high speeds and increases the power assist as the vehicle speed decreases. Here again, this system requires a vehicle speed signal as an input to the system.
A need exists for a power steering system which supplies variable steering assist in response to vehicle speed/fluid input pressure variations, yet which does not require a separate vehicle speed input signal to operate. Such a system would provide optimum road feel characteristics and power steering assist and yet would be simpler and more economical to implement.